This hasn't been said in this thread, but it seems important: The way sliders work now is that a hit circle is displayed on each endpoint on top of the slider track. For transparent sliders this means that it's impossible to have the same color for both the slider body and the endpoints. It would be great if there was a way around this problem. For example, have separate hit circles for sliders.
Secondly, I'd love to see transparency for slidertype 1. I like slidertype 1 and while it seems to have fallen out of favor, it still fits well with certain skin styles.